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Frame wobble - custom built bike



 
 
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  #1  
Old February 24th 05, 11:58 PM
jcb1973
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Default Frame wobble - custom built bike

I've recently had a Roberts hybrid built - it's suffering from frame
wobble/shimmy. It's a custom built Reynolds 725 hybrid, based on the
Audax Compact (see http://www.robertscycles.com/products.html). FWIW,
I'm 5'11", 84kg. Wheels are 36 spoke XTR hubs with Mavic T520 rims,
Conti Travel Contact tyres.

My question is - is frame wobble acceptable on an expensive custom
built bike? Roberts have tried adjusting the headset (which didn't
help). Is this just bad luck - am I stuck with it? How far should
Roberts go in trying to put this right?

Thanks for any advice - I genuinely don't know what I should expect
from the framebuilder on this.

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  #2  
Old February 25th 05, 01:37 AM
Kenny
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Default

When the steering begins to wobble press your knee against the top
tube. Experiment with your seat position. Try to put less weight on
your rear wheel.

  #3  
Old February 25th 05, 01:48 AM
Ted
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"Kenny" wrote:

When the steering begins to wobble press your knee against the top
tube. Experiment with your seat position. Try to put less weight on
your rear wheel.


Or more weight.

It's a harmonic situation; pretty much any structure has a frequency at
which kinetic energy inputs from the rider or the road surface will
cause and/or increase an oscillation. So the fix is to change the
frequency of oscillation. Any number of things will do that, and it's
easy to alter the weight distribution of the rider, as Kenny pointed out
above.

If you habitually carry stuff with you, try shifting it from, for
example, from the back to the front.

--
Ted Bennett
Portland, OR
  #4  
Old February 25th 05, 01:57 AM
Mark Hickey
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"jcb1973" wrote:

I've recently had a Roberts hybrid built - it's suffering from frame
wobble/shimmy. It's a custom built Reynolds 725 hybrid, based on the
Audax Compact (see http://www.robertscycles.com/products.html). FWIW,
I'm 5'11", 84kg. Wheels are 36 spoke XTR hubs with Mavic T520 rims,
Conti Travel Contact tyres.

My question is - is frame wobble acceptable on an expensive custom
built bike? Roberts have tried adjusting the headset (which didn't
help). Is this just bad luck - am I stuck with it? How far should
Roberts go in trying to put this right?

Thanks for any advice - I genuinely don't know what I should expect
from the framebuilder on this.


Did you spec the geometry and/or tubing? I've had customers make some
fairly outrageous requests in terms of geometry, and have made clear
that if there are problems with the handling as a result, it's not a
"warranty issue".

Normally a customer will just know what they want in terms of the
bike's "mission statement", and I'll figure out how to optimize the
handling around the necessary "fit points" (the saddle, crank and
bars). If I did that and the bike had a significant handling issue,
I'd stand behind it 100%. If the customer laid out an odd geometry
(against my advice) and it didn't work, that'd be another thing.

I couldn't find anything about the Roberts geometry on their website,
so it's hard to say if there's anything odd about their "standard"
geometry.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame
  #5  
Old February 25th 05, 05:13 AM
jim beam
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jcb1973 wrote:
I've recently had a Roberts hybrid built - it's suffering from frame
wobble/shimmy. It's a custom built Reynolds 725 hybrid, based on the
Audax Compact (see http://www.robertscycles.com/products.html). FWIW,
I'm 5'11", 84kg. Wheels are 36 spoke XTR hubs with Mavic T520 rims,
Conti Travel Contact tyres.

My question is - is frame wobble acceptable on an expensive custom
built bike? Roberts have tried adjusting the headset (which didn't
help). Is this just bad luck - am I stuck with it? How far should
Roberts go in trying to put this right?

Thanks for any advice - I genuinely don't know what I should expect
from the framebuilder on this.

well, the good news is that you've got what should be a shimmy-resistant
wheel setup - the xtr hub has a much better bracing angle than a
standard road hub. beefy rim too.

the bad news, imo, is that unless you're running a funky rack on the
rear or some other unusual device, i think it's a function of tube set
and therefore not something that can be addressed easily without getting
a different frame. two weeks ago, i finally got around to building up a
steel bianchi vigorelli i'd bought. i got it because it was essentially
the same as my bianchi pista which is an utterly great bike & superbly
stable. the two frames come out of the same taiwanese factory, use the
same external diameter tubes, etc. they even appear to have the same
frame angles when held side by side. only real difference is the
pista's higher bb & much cheaper tube set - reynolds 520 vs. 631 on the
vigorelli. the 520 has thicker walls as evidenced by a 26.8 seat post
vs. 27.2 for the 631.

result? the vigorelli is a complete noodle & a shimmy nightmare. now,
some of the shimmy could be the wheelset. i experimented with a badly
shimmying frame a while back & was able to tune out some of the
resonance with a stiffer rear wheel [straight gauge spokes drive side &
hub with a fractionally better bracing angle] - improved the ride
substantially, but the frame still had the propensity to shimmy. same
with the vigorelli, even with a straight gauge spoked rear.

in your situation, unless you have really skinny spokes like revos, the
better bracing angle of xtr should cover any improvement vs thicker
spokes on an ordinary road hub. a different frame completely cured any
shimmy i had on my first bike, with whatever wheelset. if you order
another, i recommend a "cheaper" heavier gauge tubeset with as large a
diameter as possible on the down tube & top tube. or go aluminum. of
the 3 aluminum frames i have, all are excellently stable with no shimmy
characteristics whatsoever.

others in this thread mention ways to handle shimmy if it occurs. they
are correct and offer good advice, but imo, having to cope with the
problem in the first place is unacceptible, especially when appropriate
frame design can eliminate it.

  #6  
Old February 25th 05, 05:32 AM
Tom Sherman
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"jim beam" wrote:

...
result? the vigorelli is a complete noodle & a shimmy nightmare. now,
some of the shimmy could be the wheelset. i experimented with a badly
shimmying frame a while back & was able to tune out some of the
resonance with a stiffer rear wheel [straight gauge spokes drive side &
hub with a fractionally better bracing angle] - improved the ride
substantially, but the frame still had the propensity to shimmy. same
with the vigorelli, even with a straight gauge spoked rear....


How about wrapping the tubes in the head tube/down tube/top tube are
with carbon fiber composite to add some stiffness to the frame?

--
Tom Sherman Earth

  #7  
Old February 25th 05, 05:47 AM
jim beam
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Tom Sherman wrote:
"jim beam" wrote:

...
result? the vigorelli is a complete noodle & a shimmy nightmare.
now, some of the shimmy could be the wheelset. i experimented with a
badly shimmying frame a while back & was able to tune out some of the
resonance with a stiffer rear wheel [straight gauge spokes drive side
& hub with a fractionally better bracing angle] - improved the ride
substantially, but the frame still had the propensity to shimmy. same
with the vigorelli, even with a straight gauge spoked rear....



How about wrapping the tubes in the head tube/down tube/top tube are
with carbon fiber composite to add some stiffness to the frame?

like carbon cranks? sure, that would work. better off not bothering
with the steel in the first place if investing that kind of effort though.

honestly, i'd love to be a die-hard steel guy, but aluminum frames seem
to be so much superior in this important regard, i do all my serious
mileage on them these days. curved seat stays & carbon seat posts make
the ride just as comfortable as steel.

  #8  
Old February 25th 05, 06:11 AM
A Muzi
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Default

"jim beam" wrote:
result? the vigorelli is a complete noodle & a shimmy nightmare.
now, some of the shimmy could be the wheelset. i experimented with a
badly shimmying frame a while back & was able to tune out some of the
resonance with a stiffer rear wheel [straight gauge spokes drive side
& hub with a fractionally better bracing angle] - improved the ride
substantially, but the frame still had the propensity to shimmy. same
with the vigorelli, even with a straight gauge spoked rear....


Tom Sherman wrote:
How about wrapping the tubes in the head tube/down tube/top tube are
with carbon fiber composite to add some stiffness to the frame?


Funny. Very funny.

Sky Yaeger, Bianchi's designer actually did that!
She spec'd an aluminum welded frame with 'carbon wallpaper'
on the chainstays ( "rigid, yet flexible") as a joke -
but Bianchi booked orders for it! Dealers actually believed
it! No kidding.

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971
  #9  
Old February 25th 05, 06:24 AM
Tom Sherman
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Default

A Muzi wrote:

"jim beam" wrote:

result? the vigorelli is a complete noodle & a shimmy nightmare.
now, some of the shimmy could be the wheelset. i experimented with a
badly shimmying frame a while back & was able to tune out some of the
resonance with a stiffer rear wheel [straight gauge spokes drive side
& hub with a fractionally better bracing angle] - improved the ride
substantially, but the frame still had the propensity to shimmy.
same with the vigorelli, even with a straight gauge spoked rear....



Tom Sherman wrote:

How about wrapping the tubes in the head tube/down tube/top tube are
with carbon fiber composite to add some stiffness to the frame?



Funny. Very funny.

Sky Yaeger, Bianchi's designer actually did that!
She spec'd an aluminum welded frame with 'carbon wallpaper' on the
chainstays ( "rigid, yet flexible") as a joke - but Bianchi booked
orders for it! Dealers actually believed it! No kidding.


One of the methods of increasing shear resistance in existing elevated
roadway piers lacking (remember the section of freeway that collapsed
during the 1989 Loma Prieta earthquake) is to wrap the piers with a
fiberglass reinforced polymer (FRP) blanket.

--
Tom Sherman Earth

  #10  
Old February 25th 05, 06:52 AM
David L. Johnson
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Default

On Thu, 24 Feb 2005 20:13:37 -0800, jim beam wrote:

well, the good news is that you've got what should be a shimmy-resistant
wheel setup - the xtr hub has a much better bracing angle than a
standard road hub. beefy rim too.

the bad news, imo, is that unless you're running a funky rack on the
rear or some other unusual device, i think it's a function of tube set
and therefore not something that can be addressed easily without getting
a different frame.


This is hogwash. No one can predict which frame/wheel/component/rider
combination will shimmy, and which won't. The biggest contributer to
shimmy is that last bit, the rider. Both in terms of position/mass, and
in terms of how the rider handles the bike, it can make a huge difference.
A degree or two of "bracing angle" is totally irrelevant.


--

David L. Johnson

__o | What is objectionable, and what is dangerous about extremists is
_`\(,_ | not that they are extreme, but that they are intolerant.
(_)/ (_) | --Robert F. Kennedy


 




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